When energy companies started extracting oil from shale formations in South Texas a few years ago, they invested hundreds of millions of dollars to make the volatile crude safer to handle.

In North Dakota’s Bakken Shale oil field, nobody installed the necessary equipment. The result is that the second-fastest growing source of crude in the U.S. is producing oil that pipelines often would reject as too dangerous to transport.

Now the decision not to build the equipment is coming back to haunt the oil industry as the federal government seeks to prevent fiery accidents of trains laden with North Dakota oil. Investigators probing crude-by-rail accidents, including one a year ago that killed 47 people in Quebec, are trying to determine why shale oil has proved so combustible—a question that has taken on growing urgency as rail shipments rise.

Only one stabilizer, which can remove the most volatile gases before transport, has been built in North Dakota and it hasn’t begun operation, according to a review by The Wall Street Journal.